A generator set includes a combination of a generator and a prime mover, for example, a combustion engine. As a mixture of fuel and air is burned within the engine, a mechanical rotation is created that drives the generator to produce electrical power. Ideally, the engine drives the generator with a relatively constant torque and speed, and the generator accordingly produces an electrical power output having relatively constant characteristics (frequency, voltage, etc.). However, a load on the generator, and subsequently the engine, can be affected by external factors that are often unpredictable and cannot always be controlled. And, changes in load can affect operation of the engine and generator and cause undesirable fluctuations in characteristics of the electrical power output.
For example, when an external electrical load is applied suddenly to the generator, the generator will attempt to provide for the increase in electrical power demand by drawing more mechanical power from the engine and converting the additional mechanical power to electrical power. As a result of the increased mechanical load, the engine may lug (i.e., the engine may slow as a torque load increases) until additional fuel and air can be directed into the engine, and the engine can begin producing the higher output of mechanical power required by the generator. Similarly, when an electrical load is suddenly removed from the generator, the generator will quickly reduce its electrical power production by drawing less mechanical power from the engine. As a result of the decreased mechanical load, the engine may overspeed until the fuel and air directed into the engine can be reduced, and the engine produces a lesser amount of mechanical power. As a result of the engine lugging or overspeeding, characteristics of the electrical power produced by the generator nay fluctuate undesirably.
Historically, attempts to smooth fluctuations in the characteristics of the electrical power produced by a genset have included feedforward control. Specifically, there exists a time lag between when a change in electrical load is applied to the generator and when the corresponding change in mechanical load is actually accommodated by the engine. If the change in electrical load can be sensed soon enough after its application to the generator, a signal indicative of an impending mechanical load change can be directed to the engine before that mechanical load change causes the engine to operate undesirably. In this manner, the engine may be given time to respond to the impending mechanical load change prior to the mechanical load on the engine actually changing. This forewarning may help reduce a magnitude of engine lugging or overspeeding and, subsequently, of the electrical power characteristic fluctuations.
Although feedforward control has been shown to reduce lugging or overspeeding of a genset engine, it may still be improved upon. That is, the forewarning provided by feedforward control may be inadequate in some situations for the engine to fully respond to the impending load change. As a result, the engine may still lug or overspeed undesirably and, hence, the electrical power characteristics may still fluctuate undesirably. Thus, a new control is desired that further reduces the likelihood and magnitude of lugging or overspeeding as the result of an electric load change.
One attempt to provide such control is disclosed in U.S. Pat. No. 7,098,628 (the '628 patent) issued to Maehara et al. on Aug. 29, 2006. In particular, the '628 patent discloses a generation control system for a vehicle that includes an AC generator driven by an engine, a load current detector, a driving-torque-increase calculator, a field current control means, and an engine power adjusting means. During operation, the driving-torque-increase calculator calculates a predicted increase in driving torque required from the engine by the AC generator to provide for an increase in the current supplied to an electric load as detected by the load current detector. When the predicted increase in driving torque is greater than a predetermined value, the engine power adjusting means adjusts engine power according to the predicted increase. While engine power is being adjusted, the field current control means limits an increase rate of the generator's field current within a predetermined value. In one embodiment, the field current is limited until the engine attains a predetermined speed at the increased driving torque. In another embodiment, the field current is limited until a preset time passes after the engine power is adjusted. By limiting the field current during adjustment of engine power, the likelihood of engine lugging or overspeeding may be minimized.
Although the '628 patent may help minimize the likelihood of engine lugging or overspeeding, it may still be problematic. Specifically, because the field current is limited during the engine power adjustment, the electric power provided by the generator at that time may have undesired characteristics. And, because the engine power adjustment does not commence until after the change in electric load has already been applied to the generator, the duration of the less-than-desired electrical power output may be substantial.